1. Field of the Invention
This invention relates to dispersion compensation in optical fibre transmission.
2. Discussion of the Background
Data transmission in optical fibres is generally limited by power loss and pulse dispersion. The advent of erbium-doped fibre amplifiers (EDFAs) has effectively removed the loss limitation for systems operating in the third optical communication window (around a wavelength of about 1.55 xcexcm (micrometer)), leaving pulse dispersion as a serious limitation, especially in future high-capacity multi-wavelength optical networks.
More importantly, most fibre which has already been installed for telecommunication links (ie. standard non-dispersion shifted fibre) exhibits a dispersion zero around 1.3 xcexcm and thus exhibits high (about 17 ps/nm.km (picosecond per nanometer-kilometer)) dispersion around 1.55 xcexcm. Upgrading this fibre to higher bit rates involves the use of EDFAs and a shift in operating wavelength to 1.55 xcexcm where dispersion-compensation becomes a necessity.
Several techniques have been demonstrated including laser pre-chirping, midspan spectral-inversion (phase-conjugation), the addition of highly-dispersive compensating fibre and chirped fibre gratings. Chirped fibre gratings are of particular interest, since they are compact, low-loss and offer high negative-dispersion of arbitrary and tunable profile.
When one or more chirped fibre gratings are used to provide dispersion compensation in an optical fibre link, they introduce an associated optical signal loss. It has therefore been proposed that the grating(s) should be used at an input end of an optical fibre link, preferably followed by an optical amplifier, so that the optical power launched into the link can be restored to a desired level after pre-compensation by the chirped grating(s). This previous proposal makes use of the fact that dispersion compensation of an optical pulse launched into a dispersive optical fibre link can be applied as a pre-compensation before the pulse enters the link because the dispersion and dispersion compensation are linear effects.
It is a constant aim to improve dispersion compensation techniques in optical fibre transmission systems, and particularly in systems which have already been installed using the so-called standard telecom fibre defined above. In such installed systems, any improvements must be made by discrete components rather than by using a different type of fibre for the transmission link.
Optics Letters Vol. 19, No. 17, 1994, pp 1314-1316 is an early paper on dispersion compensation using chirped fibre gratings. No teaching is given on the position of the gratings in the link to be compensated. EP-A-0 684 709 discloses a dispersion compensation arrangement using dispersion-compensating optical fibres.
This invention provides optical transmission apparatus for transmission of optical signals at an optical wavelength of about 1550 nanometers, the apparatus comprising:
a single mode optical fibre link formed at least in part of optical fibre having substantially zero dispersion at an optical wavelength of about 1300 nanometers and a dispersion of about 17 picoseconds per nanometer-kilometer at an optical wavelength of about 1550 nanometers; and
one or more dispersion compensating chirped optical fibre gratings, the aggregate dispersion of the chirped optical fibre gratings substantially compensating for the dispersion of the optical fibre link;
characterised in that the one or more gratings are coupled to the optical fibre link at respective positions substantially symmetrically disposed about the longitudinal centre of the optical fibre link.
The invention recognises that there is a difference in the performance of an optical link of the so-called standard telecom fibre when used at 1550 nm, depending on the positioning of the gratings. This difference is not predicted by the previously accepted linear effect of dispersion compensation. In such a linear system, it would not matter where the gratings were positioned.
A physical reason for the dependence on grating positioning is that the peak optical power at positions along the fibre link varies in dependence on where the dispersion compensation is performed. This peak optical power then determines the non-linear response of the fibre links.
As optical pulses propagate along an optical fibre, the fibre dispersion causes different wavelengths to travel at different velocities. The effect of this on a single gaussian pulse would simply be to broaden the pulse in the time domain, thus lowering the instantaneous peak power. However, in a train of pulses (as in a real communications link), each individual pulse can be time-broadened into the bit period for an adjacent pulse, so there can be times when, locally, the peak optical power actually exceeds the peak power launched into the fibre.
By counter-intuitively distributing the dispersion compensation at intervals along the fibre, the maximum dispersion of the pulse train at any point alone the fibre link is lower than if the dispersion compensation were provided simply at the input or output of the link. This leads to a lower maximum local peak optical power along the link, and so to an improvement in non-linear distortion along the link. Ultimately, this improvement can lead to longer (or cheaper) links being possible for the same dispersion compensating and amplifying components.
This invention therefore goes against conventional teaching, which would suggest that if the optical pulses are pre-dispersed, the optical power would be xe2x80x9csmeared outxe2x80x9d and so the peak optical power would be reduced. In fact, it has been found that the opposite can happen.
The invention is also distinct from systems employing, for example, lengths of dispersion compensating optical fibre as dispersion compensating devices. In such previous systems, fibre non-linearity has not been regarded as a problem.
It will be appreciated by the skilled man that the exact positioning of components in an optical fibre link may depend on the ease of access to the link (which may, for example, be underground) and so components may be positioned within, say, 10% of the approximate positions specified above while still falling within the advantageous arrangement of embodiments of the invention. Similarly, the skilled man will appreciate the slight latitude in the definition of xe2x80x9cstandardxe2x80x9d telecom fibre used above.
Preferably each such grating is connected to the link by an optical circulator.
Apparatus according to the invention is particularly usefully employed in optical communication apparatus also comprising: an optical transmitter connected to an input end of the optical transmission apparatus, the optical transmitter being operable to generate optical signals at an optical wavelength of about 1550 nanometers and an optical receiver connected at an output end of the optical transmission apparatus, the optical receiver being operable to receive optical signals at an optical wavelength of about 1550 nanometers.